Recently, as the integration of semiconductor devices has increased, the structure of semiconductor devices has become multi-layered. Accordingly, a polishing process for planarizing one or more layers of a semiconductor wafer is typically part of the process of fabricating semiconductor devices. The chemical mechanical polishing (CMP) process has been widely adopted as such a polishing process.
The CMP process is a process for polishing a surface of a wafer coated with, for example, tungsten, oxide, etc. The CMP process employs mechanical friction as well as a chemical abrasive to polish a surface. Mechanical polishing polishes a surface of a wafer using friction between a polishing pad and the surface of the wafer by rotating the wafer while the wafer is fixed on a rotating polishing head and while the wafer is pressed against a polishing pad. Chemical polishing polishes the surface of the wafer using slurry supplied between the polishing pad and the wafer as a chemical abrasive.
The CMP polishing process may achieve high planarity not only within a narrow region, but also over a wide region. Therefore, the CMP process is considered most appropriate as wafers become wider.
A typical CMP apparatus for performing a CMP process is shown in FIG. 1. The apparatus of FIG. 1 includes a polishing station 110 and a polishing head 120. The polishing station 110 includes a turn-table 114 provided with a polishing pad 112. The polishing head 120 includes, as shown in FIG. 2, a planar member 122, a protective guard 124, and a membrane 126.
A plurality of fine holes 122′ (e.g., twenty-eight fine holes 122′) are formed in the planar member 122, in, for example, the manner shown in FIG. 3, (i.e., in concentric circles about a central, vertical axis of the planar member 122). The fine holes 122′ are connected through fluid lines 150 to a vacuum generator 130. The vacuum generator generates a vacuum pressure for loading a wafer W′. The fine holes 122′ are also connected to a gas supplier 140 which supplies pressurized gas (e.g., Nitrogen gas) while polishing the wafer W′.
In FIG. 1, the reference numeral 152 denotes an arm connected to the polishing head, and the reference numeral 154 denotes a slurry supplier.
In the chemical mechanical polishing apparatus of FIG. 1, the wafer W′ is loaded by the vacuum pressure generated by the vacuum generator 130. The wafer W′ is polished while the wafer W′ is pressed against the polishing pad 112 due to a bulging of the membrane 126 caused by the gas pressure supplied from the gas supplier 140.
However, when a wafer W′ polished with such a chemical mechanical polishing apparatus is examined, the wafer W′ typically exhibits non-uniformly polished portions D in a circular stripe pattern as shown in FIG. 4. These non-uniformly polished portions D will hereinafter be called circular stripe portions.
When the polishing uniformity is deteriorated by such circular stripe portions D, a semiconductor device fabricated with the polished wafer may operate abnormally.
As a simple solution for such non-uniform polishing, a wafer may be polished while not pressing it to the polishing pad. However, the polishing speed is reduced when such a solution is employed. Accordingly, the productivity of the wafer manufacturing process is likewise reduced.